Nitrogen is often the rate-limiting element in plant growth and all field crops have a fundamental dependence on inorganic nitrogenous fertilizer. Since fertilizer is rapidly depleted from most soil types, it must be supplied to growing crops two or three times during the growing season. Nitrogenous fertilizer, which is usually supplied as ammonium nitrate, potassium nitrate, or urea, typically accounts for 40% of the costs associated with crops such as corn and wheat. It has been estimated that approximately 11 million tons of nitrogenous fertilizer is used in both North America and Western Europe annually, costing farmers $2.2 billion each year (Sheldrick, 1987, World Nitrogen Survey, Technical Paper no. 59, Washington, D.C.). Furthermore, World Bank projections suggest that annual nitrogen fertilizer demand worldwide will increase from around 90 million tons to well over 130 million tons over the next ten years. Increased use efficiency of nitrogen by plants should enable crops to be cultivated with lower fertilizer input, or alternatively on soils of poorer quality and would therefore have significant economic impact in both developed and developing agricultural systems.
Using conventional selection techniques plant breeders have attempted to improve nitrogen use efficiency by exploiting the variation available in natural populations of corn, wheat, rice and other crop species. There are, however, considerable difficulties associated with the screening of extensive populations in conventional breeding programs for traits which are difficult to assess under field conditions, and such selection strategies have been largely unsuccessful.